This invention relates to hydroformylation processes. In one aspect, the invention relates to a process for the mitigation of catalyst inhibition and ligand degradation.
In the rhodium/phosphite-catalyzed hydroformylation of olefins, the hydrolyzable ligand slowly and inexorably degrades to a number of by-products. Some of these by-products are acidic, and may be removed by contacting the reaction solution with an aqueous buffer (e.g. sodium phosphate). The resulting metal salts are appreciably soluble in water, and can be effectively removed, e.g. by extraction, from the organic phase. This extraction process is described in U.S. Pat. No. 5,741,944.
An additional problem inherent to rhodium-phosphite hydroformylation processes involves a loss of catalytic activity due to the formation of a class of diorganophosphite by-products. These compounds can coordinate to the active rhodium-phosphite catalyst and form new complexes that are less reactive. Fortunately, these diorganophosphite by-products may be preferentially hydrolyzed by contacting them with a buffer as described in U.S. Pat. No. 5,741,944.
Phosphate salts are recognized in the art as being preferred buffers. However, environmental agencies in some geographies have placed strict limits on the phosphorous content of plant effluent streams.
The preferred pH range of the aqueous buffer employed in U.S. Pat. No. 5,741,944 is 6-8. One set of phosphorous-free, buffer salts that buffer in this range are the maleates and fumarates. However, it has been reported that some α,β-unsaturated carbonyls are catalyst inhibitors (see U.S. Pat. No. 4,861,918, U.S. Pat. No. 4,221,743 and EP 0 306 094 B1). Since these species can coordinate to rhodium in a bidentate fashion via their olefin and carbonyl moieties, it is generally believed that they inhibit the reaction by forming inactive complexes until such time as they are slowly reacted off the catalyst and the active site is thus made available for reaction. For example, hydroformylating acetylene gives acrolein, an α,β-unsaturated aldehyde product that inhibits the catalyst until it is subsequently hydrogenated or hydroformylated to a bis-aldehyde (see U.S. Pat. No. 5,675,041 and WO 2010/030339). In another example, U.S. 2011/0028746 describes this type of interaction in a Rh-catalyzed decarboxylative hydroformylation, wherein extremely high concentrations of Rh (4600 ppm) were required. A list of expected rhodium hydroformylation poisons was tested in GB 1,497,627.
U.S. Pat. No. 5,466,644 and U.S. Pat. No. 4,283,304 teach the addition of maleic acid to destroy phosphorous-based ligands via a Michael addition reaction. In particular, '304 teaches that washing out any residual maleic acid is desirable, especially for commercial operations. Based on these two patents, a continuous extractor process, which would be expected to have traces of maleate going into the reaction system, would be expected to significantly impact ligand degradation via the Michael addition reaction. In systems with expensive ligands, increased ligand decomposition is not commercially acceptable.
It would be desirable to have an effective non-phosphorous based buffer that buffers in the range of pH 6 to 8 and is not detrimental to the hydroformylation process.